Consider using vlib_log for logging

[ivan4th]

Some of the diagnostics that can only be displayed by building UPG with various values of CLIB_DEBUG should be instead done using macros that use vlib_log() under the hood, using debug log level. This way, it will be possible to get extended diagnostics by using set logging class CLI command.

Care should be taken not to affect the performance in some tight loops, though; in these places, the old scheme can be left in place or we might add extra checks for log levels before doing the calls.

We may use several logging classes (charging, forwarding, etc)